The present invention relates to instrumentation for spinal deformities and more particularly to spinal hooks, a support rod and locking structure for compressing and/or distracting the spine.
Scoliosis is a lateral deviation of the spinal column. When the spinal curvature exceeds a given limit it becomes necessary to consider surgical treatment. Such surgical treatment is known as arthrodesis and consists of fusing together the vertebrae of the scoliotic curvature after correcting that curvature to the maximum possible extent. Correction can be accomplished prior to surgical treatment by continuous traction of the spine or by corrective plaster casts.
During surgery the correction is completed and finalized. For this purpose, a threaded support rod and hooks are placed on the spinal column for the purpose of straightening the spine and maintaining the correction until arthrodesis is accomplished by means of autogenous bone graft. The heretofore known implants used to correct curvature during surgery include the Harrington system and the system disclosed in U.S. Pat. No. 4,269,178 to Keene, granted May 26, 1981.
Each of the Harrington compression and distraction systems comprises a threaded metal rod and a plurality of hooks having longitudinal passageways therein through which the rod extends. The hooks slide freely along the metal rod and each is adjusted by an internally threaded nut that mates with the rod. Appropriate placement of the hooks on the rod and tightening of the nut affects either compression of the convexity of scoliotic curvature or distraction of the concavity of such curvature. In the Harrington system, the individual hooks are positioned on the threaded rod by inserting either the top or bottom end of the rod into the longitudinal passageway extending through each of the hooks.
The system disclosed in Keene's U.S. Pat. No. 4,269,178 is different from the Harrington system in that it allows special hooks to be tilted and manipulated into position between the vertebrae before the threaded rod engages the hooks. Each of the hooks includes a slot in the rear surface thereof that communicates with the longitudinal passageway within which the threaded rod rests. While this arrangement allows easy placement of the hooks on the rod without threading them onto the rod from the rod ends, a sleeve interlock is required that prevents the threaded rod from being withdrawn out through the slotted opening in the rear surface of the hook. German Auslegeschrift No. 2,649,042 discloses a similar interlock wherein bone screws are prevented from post-operative displacement from either rear or side slotted openings in the screw heads.